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© 2012 AOAC INTERNATIONAL
AOAC O
FFICIAL
M
ETHODS
OF
A
NALYSIS
(2012)
M
ICROBIOLOGY
G
UIDELINES
Appendix J, p. 17
ANNEX E
Example of Graph of POD Values from
Qualitative Method Single Laboratory Data
Notes
:
(
1
) The concentration plotted should be MPN/test portion.
(
2
) Confidence intervals may also be plotted.
(
3
) Collaborative data should be plotted analogously.
ANNEX F
Calculation of LPOD and dLPOD Values from
Qualitative Method Collaborative Study Data
For a multilaboratory trial where L = number of laboratories, R =
replicates per laboratory, N = LR = total replicates, LPOD estimate
is given by
where
x
is the number of positive results.
Method for estimating LPOD 95% confidence intervals:
Step 1.
—Enter data into AOAC spreadsheet with 1 for positive
response and 0 for negative response. Record the mean LPOD,
s(R), and s(r).
Step 2
.—Calculate s(L), standard deviation due to laboratory
effect as:
2
2
( )
( )
( )
s L s R s r
Step 3
.—Calculate s(POD) as the standard deviation of the
individual laboratory POD estimates.
2
(
)
(
)
1
i
POD LPOD
s POD
L
¦
Step 4
.—Calculate degrees of freedom, df for s(POD) as follows:
2 2
2
2 2
2 2
( )
( )
1
s r
s L
L N
df
s r
s L
N
L
L
N L
ª
º
«
»
«
»
¬
¼
ª
º
ª
º «
»
«
» «
»
¬
¼ ¬
¼
Step 5
.—Calculate 95% confidence limits on LPOD:
If 0.15 ≤ LPOD ≤ 0.85:
ANNEX D
Summary Data Table for Qualitative Method Single Laboratory Studies
Table D1. Comparative results for the detection of
Listeria innocua
in raw shrimp
Concentration
Candidate presumptive (CP)
Candidate confirmed (CC)
Candidate method (C)
Reference method (R)
C vs R CP vs CC
Statistic
MPN/25 g
N
x POD(CP)
N
x POD(CC)
N
x POD(C)
N
x POD(R)
dPOD(C,R) dPOD(CP,CC)
Estimate
0.00
20
0
0.00
20
0
0.00
20
0 0.00
20
0 0.00
0.00
0.00
LCL
0.00
0.00
0.00
0.00
0.00
–0.16
–0.16
UCL
1.50
0.16
0.16
0.16
0.16
0.16
0.16
Estimate
0.80
20
12 0.60
20
10
0.50
20
10 0.50
20
11 0.55
–0.05
0.10
LCL
0.43
0.39
0.30
0.30
0.34
–0.33
–0.19
UCL
1.39
0.78
0.70
0.70
0.74
0.24
0.37
Estimate
3.00
20
20 1.00
20
20
1.00
20
20 1.00
20
19 0.95
0.05
0.00
LCL
1.58
0.84
0.84
0.84
0.76
–0.12
–0.16
UCL
5.68
1.00
1.00
1.00
1.00
0.24
0.16
Estimate
17.00
20
20 1.00
20
20
1.00
20
20 1.00
20
20 1.00
0.00
0.00
LCL
0.27
0.84
0.84
0.84
0.84
–0.16
–0.16
UCL
1060
1.00
1.00
1.00
1.00
0.16
0.16